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Connecting Pushrods

Old 12-11-2002, 09:19 PM
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airhead
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Default Connecting Pushrods

Well ... I am to the point of running the pushrods through their sheaths and simply connecting them to the control surfaces. The question I have is simply the following -

What is the best way for measuring out the rods?? (e.g. should they be neutral? pushed all the way forward? pushed all the way backward?)

Thanks!

Dan
Old 12-11-2002, 10:34 PM
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FHHuber
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Default Connecting Pushrods

Before cutting the nyrod OR the metal ends...

Ensure that the sheath is FIRMLY anchored, preferably at least every 4 inches (or every bulkhead in the plane). There should be an anchor "bar" across the interior of the fuselage, just clearing the servo screws, if the srvos are more than 1 inch from the closest bulkhead supporting the sheath. If the sheath moves, the rod will flex and that means the controls will be sloppy.
(I usually cover the fuselage AFTER installing the sheaths... If its a slab sided fuselage with no lightening holes, the sheaths go in before the bottom is sheeted.)

Insert the unthreaded length of metal rod into the nyrod inner core, and thread the nylon onto the threaded portion at least 4 turns. This is the end which exits the rear of the fuselage.

Install the servos and turn on the TX and RX. (no wheels or arms installed!) Center the throttle, and center all trims. Then turn the RX off before turning off the TX. (servos are all centered and you can trust the rods to be within 1/8 inch of final correct length when you cut them)

Install control horns wth the holes for the control rods/clevices as close to being directly in line with the hinge lne as possible.

Pin the control surfaces in the neutral position.

Press output arms onto the servos with the arms perpendicular to the fuselage centerline.

Screw a clevice on the exterior end of each rod. Engage a minimum of 6 turns, preferably 15 turns.

Insert the pushrod and connect to the control horn.

Affix a clevice to the servo output arm, and hold it in position next to the nyrod, mark 1/8 inch down the rod. Remove the Nyrod from the plane. Cut on the mark. Insert the 1.5 inch all-threaded rod such that 5/16 to 3/8 inch remains outside of the nylon.

Reinsert the nyrod, screw the interior clevice onto the rod such that 1/8 inch of threads are visible between the clevice and the nylon.


For engine... set the carb at appx 50% throttle.

Don't forget to put "keeper screws" on each servo output arm...

Test all controls for appropriate throws.
Old 12-12-2002, 07:10 AM
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airhead
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Default Connecting Pushrods

Thanks much!!!

- Dan
Old 01-31-2003, 02:57 PM
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redneck
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Default Connecting Pushrods

If those inner pushrods are plastic, I'd throw them away and opt for solid wire pushrods, they will slide in the outer sleeve fine. The plastic pushrods expand and contract with temperature changes and you'll be trimming your control surfaces much more than normal. The solid wire pushrods add a little weight, but I believe they're worth it for the added convenience.
Old 01-31-2003, 11:49 PM
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FHHuber
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Default Connecting Pushrods

I've run nyrods for years... no trimming problems. The trimming problems come from improperly secured outer sheathing.

I even used the "notorious" rods that came with the 4*40. Properly mounted... they don't have thermal expansion problems. I can't find a difference in trim setting on a 40 deg day vs. a 110 deg day.
Old 02-01-2003, 10:18 AM
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RONSTERMONSTER
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Default Connecting Pushrods

I use carbon fiber rods and they donot have any trim changes with temp . I had a hog bipe with plastic rods and had to trim every time ther was a drastic temp change . The outer rod was captured every 3 or 4". The carbon fiber is as flexable as plastic not any heaver and only a little more costly .
Ron
Old 02-02-2003, 10:05 AM
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redneck
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Default Connecting Pushrods

FHHuber,
I was wondering why you referred to the nyrods that come with the 4-star as "notorious" ?? Hmmm...
Old 02-02-2003, 11:32 AM
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Default Connecting Pushrods

I referred to the 4*40 rods specificly because... they are the ones people seem to complain about the most as being subject to expansion/contraction with heat/cold.

The error I see most commonly with flexible plastic/nylon pusrods is having 1 to 4 inches of unsupported plastic rod between the fueslage exit and the control horn. (or similar unsupported sections between the TE bulkhead and the servo) The unsupported sections FLEX differently at different temperatures and are sometimes flexible enough in hot weather that you can't get any push at all...

The end sections need support either from added guides, or internal wire. I make sure that a minimum of 1/2 inch of wire is in the fuselage with the rod pushed for maximum deflection.
Old 02-02-2003, 11:39 AM
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Rodney
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Default Connecting Pushrods

I tend to agree with Mr. Huber, no problem with temperature and nylon if you properly install them like Mr. Huber recommended. True, if you go from -20 degrees to +120, you might have to use a click or two of trim.
Old 02-05-2003, 02:13 PM
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redneck
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Default Connecting Pushrods

O.K. Lets say I put my Sig LT-40 (equipped with nyrod pushrods) away after flying. All my trims are near perfect. The next time out its 40 degrees colder. I set my plane on the ground and notice that my elevator is pointing down at about 15 degrees from level, and my rudder is turned as well (nose gear is straight). I turn on my radio and plane expecting them to go back to a more neutral position (maybe they have just been bumped huh?). But nothing happens. Next I check all my trim tabs and they haven't been moved. My buddy tells me it due to thermal expansion (sounds viable huh?), so I replace them both with solid wire pushrods. I have never had any more trouble like this. Not trying to be smart here, but if thats not thermal expansion, I would like for you to explain in more detail how support of the outer tubes effects this condition.
Old 02-05-2003, 04:21 PM
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Default Connecting Pushrods

The expansion rate of nylon just isn't high enough to give the severe deflection you are trying to illustrate. It CAN'T happen, especially in the minor temp changes from one week to another.

That large of a control position change is indicative of something major wrong in the mounting of servos or something else. I'd ground the plane until it was located and resolved. Nyrods won't cause it. If the percentage expansion of nylon was that bad... they wouln't make nylon pushrods at all.

Maybe in the changing from the nyrods to solid wire you cured some other problem you hadn't noticed... such as unsupported pushrod section flexing. the difference n flexibility of the nylon at 80 deg vs 40 deg is huge! That 4 to 6 inch section between the control horn and the fuselage entry could make the difference you indicated due to flexing in a different curve at the different temperature.

Just the indication of the nosewheel being straight and the rudder being off is obvious evidence of an improperly installed control system. It CAN'T happen if you do it right.
Old 02-07-2003, 12:38 PM
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redneck
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Default Connecting Pushrods

FHHuber,
Thanks for all your help. My solid wire pushrods are still functioning flawlessly! In my first few years of studying engineering, I learned not to accept "it can't happen".
Old 02-07-2003, 02:38 PM
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redneck
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Default Connecting Pushrods

Airhead,
I apologize for getting off the subject with FHHuber. Maybe I should start my own thread on expansion, contraction of nyrods. I'd like to get some more opinions. Good Day !

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